This chapter explores several features of the IOBL by combining observations and modeling based on local turbulence closure as incorporated into a numerical model described in Chapter 7. The intent is to elucidate certain features of the response of the upper ocean to variations in forcing that require consideration of the time dependence of the physical conservation equations.
First, we show that an interesting series of upper ocean measurements at the SHEBA site near the time of maximum insolation, when there was a clearly discernible diurnal signal in both temperature and downward turbulent heat flux at two measurement levels, can be adequately simulated. However, the simulation makes sense only if solar radiation penetrating the compact ice cover is significantly greater than has been typically assumed in the past.
Next is a simulation of events observed in late summer at the SHEBA site, when there was energetic inertial motion of the ice and upper ocean. Inertial oscillation nearly always implies strong shear in the upper part of the pycnocline, and early models of mixed-layer evolution (e.g., Pollard et al. 1973; Niiler and Kraus 1977) related the rate of mixed-layer deepening (“entrainment velocity”) to a Richardson number involving the inverse square of the velocity of a uniform slab of water (volume transport divided by mixed-layer depth). In the slab model of Pollard et al. (1973), for example, the velocity was inertial and any deepening was confined to the first half inertial period unless the inertial velocity increased. This was an unrealistic limitation and much effort was devoted to elaborating how entrainment would take place at the base of the mixed layer, while still retaining the simplicity of constant temperature, salinity, and velocity in the mixed layer (and the shear that this implied at the mixed-layer/pycnocline interface). Our initial measurements from the AIDJEX Pilot Experiment demonstrated convincingly that the IOBL was not “slablike” but exhibited definite and predictable shear in the IOBL. McPhee and Smith (1976, their Figs. 8.11 and 8.12) included an example during a storm where on the second day, the Ekman layer was confined to levels well above the obvious pycnocline established by stronger forcing on the first day. Nevertheless, it remains an article of faith among many oceanographers that inertially oscillating slabs are a primary mechanism by which mixed layers remain mixed. In Section 8.2 we look at this from the perspective of a model forced with different boundary conditions.
Finally, in Section 8.3 we examine a time fromthe MaudNESS project near Maud Rise in the Atlantic sector of the Southern Ocean, when the cold upper layer was very close to the same density as the underlying Warm Deep Water, despite being less saline. Here we use the model to illustrate how nonlinearities in the equation of state, and possibly, differences in thermal and saline diffusion, come into play.
KeywordsMixed Layer Friction Velocity Potential Density Turbulent Heat Flux Inertial Oscillation
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